Moony, Wormtail, Padfoot, and Prongs

Messrs. Moony, Wormtail, Padfoot, and Prongs
Purveyors of Aids to Magical Mischief-Makers
-- written on the Marauder's Map (PA10)

"Moony, Wormtail, Padfoot, and Prongs. ...We owe them so much."
"Noble men, working tirelessly to help a new generation of lawbreakers."
-- George and Fred Weasley (PA10)

Nicknames of the Marauders used amongst themselves and in the creation of the Marauders Map.

Moony = Remus Lupin, refers to being a werewolf

Wormtail = Peter Pettigrew, refers to being a rat Animagus

Padfoot = Sirius Black, refers to being a black dog – also another name for The Grim

Prongs = James Potter, refers to the antlers of his stag Animagus

Harry first sees the names written at the top of the Marauder's Map in "great, curly green words" in response to a tap from his wand and the incantation " "I solemnly swear that I am up to no good" (PA10)

When Snape catches Harry in the corridor at night with the Map, he tries several times to get it to reveal it's purpose until each member of the Marauders uses the to insult him in writing:
"Moony presents his compliments to Professor Snape, and begs him to keep his abnormally large nose out of other people's business."
"Mr. Prongs agrees with Mr. Moony and would like to add that Professor Snape is an ugle git."
"Mr. Padfoot would like to register his astonishment that an idiot like that ever became a professor."
"Mr. Wormtail bids Professor Snape good day, and advises him to wash his hair , the slimeball." (PA14)

The story of the Marauders was finally told to Harry by Remus Lupin in the Shrieking Shack, when he explained that the four Gryffindor friends used nicknames (PA18). The map insulted Snape because he had been their childhood nemesis, sneaking around trying to discover if Lupin was a werewolf. To make matters worse, Padfoot (Sirius Black) had played a trick on Snape, somehow luring him to the Whomping Willow on a full-moon night when Lupin had transformed, which could have been fatal for Snape. James discovered the plot and saved Snape at the last minute, and Dumbledore made Snape promise not to tell Lupin's secret, and he kept his promise except with Lily Evans, whom he tried to warn (DH33).